Pumping systems, and particularly, systems comprising a vacuum pumping mechanism, are used extensively in semiconductor processing systems. A typical pumping system 60 is shown in FIG. 7. Processing of semiconductor wafers is performed in a vacuum, or processing, chamber 62. The pressure of process gas in the processing chamber during processing is maintained at relatively low processing pressures by a vacuum pump, or pumping mechanism, 64. The pressure is typically kept at processing pressures for extended periods and is allowed to return to atmospheric pressure periodically for repair and maintenance of equipment, for example. Unprocessed wafers are introduced to the processing chamber and processed wafers are withdrawn from the processing chamber via a load lock chamber. The pressure in the load lock chamber is controlled by a vacuum pump such that wafers can be transferred to and from the semiconductor processing system when the load lock chamber is at atmosphere, and wafers can be transferred between the load lock chamber and processing chamber or chambers when the load lock chamber has been evacuated to processing pressures. When choosing a vacuum pump for a chamber associated with a semiconductor processing system, the power requirement of the pump must be specified. A drive control 50, including a variable speed drive is chosen with an appropriate power requirement for controlling a motor 51 of the pump. Typically, the power requirement for a drive and a pump will be the same.
A vacuum pump is required, therefore, to be able (a) to reduce the pressure in a chamber to processing pressures and (b) to maintain processing pressure in a chamber. When a vacuum pump maintains processing pressures in a chamber, it is resisting atmospheric pressure from flowing into the chamber from downstream of the pump. This is termed operating at ultimate. Operating at ultimate is relatively less demanding on the pump's power requirements. Operation of the pump to evacuate a chamber from atmosphere to processing pressures (pump-down) requires relatively more power.
It is generally the procedure to select the power capacity of a vacuum pump, and also the drive, to meet the requirement at pump-down, even though pump-down may only take, for example, 2-3% of the vacuum pump's active life. The cost and size of the pump's drive increases with increased power requirement, even though increased power is required for only a small percentage of operation.
Referring to FIG. 1, a prior art drive control is shown which includes a variable speed drive 50 for controlling the power delivered to a motor 51. Drive 50 comprises a first module 52 for monitoring a motor thermal load (MTL) of the motor. As is known in the art, the motor current (Imotor) is input to the first module, which estimates motor thermal load. Irated is a rated current at which the motor can operate indefinitely without overheating. The first module calculates the square of Imotor over Irated and uses a first order low-pass filter 52 (with a time constant τ and the Laplace operator s) to calculate motor thermal load. The motor can be thermally modelled using the first order system with its temperature being a function of the square of the input current.
The first order low pass filter represented in module 52 is digital, but alternatively, motor temperature could be modelled by analogue means. A higher order filter could be used for greater accuracy.
The second module 54 comprises a current control module 56 for transmitting electrical power to motor 51 as indicated by the arrows referenced electrical power to motor. The power is controlled by controlling the current supplied to the motor, which in turn is controlled by adjusting the frequency and/or amplitude of the voltage in the motor. A programmable internal drive current limit 58 outputs a drive current limit to control module 56 for limiting the power transmitted to the motor. A comparator 57 compares the motor thermal load MTL with a predetermined motor thermal load held in pre-set trip value store 60. If the determined motor thermal load exceeds the pre-set trip value, a trip command is transmitted to the control module 56 for cutting power to the motor. Tripping involves a sudden and immediate reduction of motor power to zero, the purpose of which is to protect the motor from damage.
In general, motors and drives can operate at 100% of their rated power indefinitely. However they can be overloaded to, typically, 200% (or more) of rated power for a short term time limited period.
FIG. 2 shows a graph of current (Imotor) against time for variable speed drive 50. Broken line 62 indicates when the motor thermal load exceeds the pre-set value and thus when tripping occurs. The broken line is plotted according to the relationship between current and motor thermal load. The rated power of a motor is the power at which the motor can be operated indefinitely without overheating and therefore without tripping. A motor operated at rated power, rated voltage and rated frequency, draws a rated current Irated referred to above in relation to FIG. 1.
FIG. 2 shows the rated current Irated as (100%), which is the current that can be sustained indefinitely without overheating the motor. As will be seen from FIG. 2, when drive control 50 is operated in an overload condition at a current of 200%, tripping of the motor occurs at a time ttrip (200%) and operation in an overload condition at a current of X % leads to tripping of the motor at a time ttrip (X %). It will be appreciated therefore that the time at which tripping occurs is dependent on the amount of the overload current (i.e. the extent to which the current exceeds the rated current).
It is possible, therefore to operate a motor in overload conditions to decrease, say, pump-down times of a vacuum pump in a semiconductor processing system. However, this has the disadvantage that the drive may trip if the overload is held too long or is too high. This is disadvantageous because if the motor is stopped, the semiconductor processing wafers may be damaged.
The present invention seeks to improve pumping system performance by deliberately operating the system for transient periods in an overload condition without any possibility of tripping.